syncing arp/table on 6500s

Answered Question
Aug 5th, 2009
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Hi all, I'm looking to set my arp timer to 420s and my cam table timer to 480 to get rid of unicast flooding. Anyone have any insight into the effect this will have on existing traffic? Does the switch accept the new timers gracefully or does it flush the tables and have to re-arp or re-broadcast to learn everything again.


Need to see whether this is something that needs to be done in a defined maintenance window or at any time.


Thanks in advance.


/rls

Correct Answer by jbrenesj about 7 years 7 months ago

All of the arp entries above the new timeout will be re-arped.


Considerations, that I can recall...


LOWER the ARP timeout:

At the moment the timer expires, the router will ARP for the IP address. In a huge network you will see more frequent arp activity but it will flush a lot faster the devices that were removed/shut from the network which is good.


Lower the CAM: in this case you are only reducing the timeout on 20sec which won't hurt the default behavior of the switch.


I always lower the ARP timer instead of increasing the MAC in order to avoid the unicast flooding problems


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Correct Answer
jbrenesj Wed, 08/05/2009 - 09:57
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All of the arp entries above the new timeout will be re-arped.


Considerations, that I can recall...


LOWER the ARP timeout:

At the moment the timer expires, the router will ARP for the IP address. In a huge network you will see more frequent arp activity but it will flush a lot faster the devices that were removed/shut from the network which is good.


Lower the CAM: in this case you are only reducing the timeout on 20sec which won't hurt the default behavior of the switch.


I always lower the ARP timer instead of increasing the MAC in order to avoid the unicast flooding problems


Peter Paluch Wed, 08/05/2009 - 13:31
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Jorgemario,


You have written: I always lower the ARP timer instead of increasing the MAC in order to avoid the unicast flooding problems.


This confuses me. Are you suggesting that lowering the ARP timeout reduces unicast flooding?


Best regards,

Peter


rsamuel37 Wed, 08/05/2009 - 14:16
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Peter, that is what he's saying and what I will be doing as well. The default arp timer is 4 hours, but the cam timer is 5 mins. With this setup, you'll get unicast flooding every 5 minutes as the cam timer ages out.


I'm setting the arp timer to 420 and the cam timer to 480. That way I ensure that L3 sends out a broadcast before it has to flood the same packets out to each interface in a given vlan because it doesn't have the L2 info.


Somebody correct me if I've misstated anything please, but this is how I see it.


/rls

Peter Paluch Wed, 08/05/2009 - 14:33
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Hello Jorgemario and Robert,


I see the logic now. Thank you very much for enlightening me!


Best regards,

Peter


rsamuel37 Wed, 08/05/2009 - 14:22
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Jorgemario, thank you for the information.


/rls

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